Mechanical Frog had a pretty good video about the basics:
https://www.youtube.com/watch?v=UWvIa61WNWwMy basic steps would be:
1. Remove the most obvious mold lines. With this PVC plastic material, the only reliable way is to cut them with a blade - they don't sand or file well, in my experience. With other materials I like using the back of an X-Acto blade.2. Wash with dish detergent and dry - this is less important with plastic than metal or resin, but getting rid of your finger oils is still a good thing.3. Prime. If one day you get an airbrush, those make this step a little nicer. With spray cans, I'd stick to black; white primers are more prone to fuzz up on you because the paint started to dry before it hit the mini. Go outside, put the minis in a box on its side, spray at ~8" distance. You should be sweeping past the minis 2-4 times and starting, changing direction, and stopping only when you're not pointing directly at them. Use a gloved hand to rotate them by their bases and you may need to pick them up to spray from below. Nitrile gloves are best. Wear clothes you don't care about and if you wear glasses, wear goggles over them because I've ruined the coating on a pair before. Finally, move the whole stinky box somewhere safe from dust but also where you won't breathe in the odor. This whole process sucks - and may not work at all in humid environments or cold winters. Priming by hand is slow, though, and often lumpy. Airbrushing is comparatively wonderful but the setup is expensive.3a. Zenithal highlighting (optional) - I warned about white primers, but a common technique is to take white or light gray (not necessarily a primer-type paint) and spray your black-primed minis only from above, or from an angle in front and above. Keep the angle consistent between minis, and maybe expand that angle if you're going for white or yellow as your final color. This is more important if you're going to use a light color. It's also necessary if you're using a Contrast/Speed Paint type of paint. They're meant to go over a fully light gray/white undercoat but keeping the lower surfaces in black primer can give a nice effect.
4. I don't remove my mechs from bases to put onto sticks. It's a good idea, but I'm used to just holding them by the base, but you can superglue them to a large cork or put them on a painting handle.
5. If you're using Contrast/Speed Paints, the Camo Specs Online YouTube videos are really good about explaining their unusual behavior. Like the Grey Death Legion Marauder video. Regular paints - take a #2 or #3 watercolor brush, dilute your paint on a palette, apply paint to the tip of your brush (just keep it away from the lower half of the bristles) and have at it. Everyone talks about trying to dilute to the consistency of milk... just play around until you find a sweet spot between smoothness and coverage. Don't expect full coverage in one coat. Cool colors do better in that regard, though. Early on, just focus on painting between the lines. There's a lot to learn about brush technique later on. Get your base color on there, then paint the metal bits (rinse in a separate cup or you might get metal flakes onto whatever other paints you're using that session) and any secondary colors.
5a. On brushes - the cheapest ones aren't pointy to begin with. The midranged ones are pointy to start, but with a little use that point starts to hook. The good ones hold a point for longer before starting to hook. Natural Kolinsky sable tends to be the best.
6. Wash with something like Army Painter's tones, GW Nuln Oil, etc. Messy washes will stain the whole mini, which you may want. Targeted washes along panel lines are slow. If you want to do that, oil washing is surprisingly fast and forgiving though it requires adding a couple of varnish coats to the process. Don't worry about it yet, but when you're ready for the next step in washing, read up on it.7. Drybrushing is the easiest highlighting technique. Get a soft makeup brush, apply undiluted paint, dry on your towel (usually a paper napkin but it has to be one that doesn't shed particles) until there's barely any left, and flick your brush over the mini in one direction (down). Camo Specs Online covers it pretty well. Later on there's edge highlighting, panel highlighting, volumetric highlighting... whole world out there.8. Details - I use a #0 brush. There are smaller brushes but the paint dries on them before you even reach the mini. A pointy #0 brush can do anything. Paint the cockpit (also something Camo Specs Online has videos about). The simple way is to paint silver or gold then apply an ink or Contrast color over it for a shiny effect.
9. Basing. Ideally you do the assembly parts of this before painting but only the diehards do that. Apply PVA glue, diluted so it's easier to work with, with a toothpick or brush you don't care about. Once it's dry, put some diluted glue over it again so nothing comes off. Put terrarium sand on it (it's finer). Paint, wash, and drybrush. Add tiny rocks or bits of fake vegetation or whatever else you can think of. Try to make it look like the mech is really stepping into the material and not floating over it.
Do this 100 times and you will be good at painting.
